linux-kernel - Re: Dealing with the NMI mess

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Date:	Mon, 7 Sep 2015 12:19:45 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	"Maciej W. Rozycki" <macro@...ux-mips.org>,
	Ingo Molnar <mingo@...nel.org>
Cc:	Borislav Petkov <bp@...en8.de>,
	Andy Lutomirski <luto@...capital.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Willy Tarreau <w@....eu>, Steven Rostedt <rostedt@...dmis.org>,
	X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Brian Gerst <brgerst@...il.com>
Subject: Re: Dealing with the NMI mess

On 07/09/2015 10:19, Maciej W. Rozycki wrote:
>> > Essentially the ICE breakpoint instruction enters SMM mode?
>  I didn't do stuff at the probe firmware level so I can't say for sure, 
> but my gut feeling is the debug mode is indeed very close if not the same 
> as SMM.  I think duplicating the logic would be an unnecessary waste of 
> silicon.

I researched SMM a bit recently in order to implement it in KVM, and the
best source of folklore seems to be http://www.rcollins.org/ddj (which I
also have on paper :)).

The author there says that SMM design was roughly based on the 386's
probe/ICE mode design, but it's actually separate.  Most notably, on the
386 the state save areas almost mirror each other, but when I say
mirror... I do mean mirror: directions are reversed, and what is on top
for probe mode is on bottom for SMM. :)

In addition, AMD tried reusing ICE mode for SMM, and was sued by Intel
who actually won the lawsuit.  I couldn't find more information about
the lawsuit.

It's probably diverged more and more over time, for example because SMM
is now considered security-sensitive while probe mode isn't.  In
addition, the same DDJ article says that Pentium JTAG probe mode
"doesn't resemble SMM at all, doesn't use a state save map, or even
execute any code of its own", whatever that means.

Paolo

>  And obviously it's any cause of #DB that enters this mode.  The probe can
> also request it right at the exit from the reset state, so that you can 
> debug software (e.g BIOS startup) right from the reset vector.  You don't 
> need working RAM for that.
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